1. Field of the Invention
The present invention relates to printers, and more particularly, to printers having a memory for storing a print image.
2. Description of Related Art
One type of printer known as a "laser" printer employs an electrophotographic print engine in which the image to be printed onto paper or other media is first formed on a charged, light-sensitive drum by a laser which scans the drum. Laser printers typically require a complex internal microcomputer system which receives printing instructions from a host computer in a high level programming language such as PostScript.TM.. These programming language instructions are processed by the printer microcomputer to produce data defining a two-dimensional image pixel by pixel. Because of the complexity of the processing task, the generation of the two-dimensional image data can be very time consuming, requiring as much as several minutes per page for images which include sophisticated graphics and complex fonts. As a consequence, much time is often wasted should the reprinting of one or more pages of a document be necessitated by a printer malfunction such as a paper jam.
Furthermore, when more than one copy of a document is required, the long processing time of the printer computer often makes it impractical to use the printer to print multiple copies. Instead, once the printer has printed one copy of the document, it is often faster to use a separate photocopying machine to produce the additional copies.
One approach for eliminating the need for a separate photocopier has been to program the printer to print multiple copies of each page of the document before processing and printing the next page of the document. Because the actual time required to print a page is relatively short (on the order of a few seconds or less), multiple copies of a page can be printed relatively quickly once the print image data of the page has been generated by the printer computer. However, the pages once printed must then be sorted and collated either manually or by a mechanical sorter to produce the multiple copies of the document. These mechanical sorters tend to be expensive and complex. In addition, because the sorter typically sorts the pages by directing individual pages into different bins, the number of copies which can be produced at one time is often limited by the number of bins. Similarly, the size of document is usually limited by the size of each bin.
To eliminate the need for mechanical sorters, it has been proposed to store into memory the print image data produced by the printer microcomputer. The stored print image data for each page is then read from the memory in a cyclical fashion so that the pages are printed in the correct order to produce multiple copies without the need for additional sorting. However, because of the high resolution of some images, the microcomputer typically requires a large amount of internal random-access-memory (RAM) for the computational tasks alone. Moreover, some sophisticated laser printers may employ a hard disk as well for handling complicated, high definition images. Consequently, memory space in printer computer systems is often at a premium.
To store the print image data for a single page typically requires a significant amount of memory space. A monochrome image at 300 dots per inch on a 81/2.times.11 inch page, can require one megabyte of memory. This memory requirement quickly escalates for color or high definition images and is multiplied by each page of the document to be printed. To meet these requirements, it has been proposed in U.S. Pat. No. 4,597,018, for example, to store print image data on a read/write magneto-optical disk. Although optical disks can store large amounts of information (typically on the order of a gigabyte or more), such memory storage devices tend to be relatively expensive and slow.